Senate Judiciary Committee Chairman Chuck Grassley said Thursday that Donald Trump Jr. and former Trump campaign chairman Paul Manafort would be subpoenaed if they did not appear before his panel next week.

"We sent the letter," the Iowa Republican told CNN at the Capitol. "I don't know whether they've accepted, but we sent the letter.

"We asked them for a voluntary appearance," he added. "At least publicly, I indicated to them that there will be a subpoena if they don't come."

Trump Jr. and Manafort are scheduled to testify before the Judiciary Committee on July 26, according to a schedule of the hearing posted on Wednesday, but they have not yet confirmed whether they would appear.

Senate Judiciary Committee Chairman Chuck Grassley said Thursday that Donald Trump Jr. and former Trump campaign chairman Paul Manafort would be subpoenaed if they did not appear before his panel next week.